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Airspeed Indicator Calibration

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TECHNICAL GUIDANCE MATERIAL AIRSPEED INDICATOR CALIBRATION This document explains the process of calibration of the airspeed indicator to generate curves to convert indicated airspeed (IAS) to calibrated airspeed (CAS) and has been compiled as reference material only. i Technical Guidance Material BushCat NOSE-WHEEL AND TAIL-DRAGGER FITTED WITH ROTAX 912UL/ULS ENGINE APPROVED QRH PART NUMBER: BCTG-NT-001-000 AIRCRAFT TYPE: CHEETAH – BUSHCAT* DATE OF ISSUE: 18th JUNE 2018 *Refer to the POH for more information on aircraft type. ii For BushCat Nose Wheel and Tail Dragger LSA Issue Number: Date Published: Notable Changes: -001 18/09/2018 Original Section intentionally left blank. iii Table of Contents 1. BACKGROUND ..................................................................................................................... 1 2. DETERMINATION OF INSTRUMENT ERROR FOR YOUR ASI ................................................ 2 3. GENERATING THE IAS-CAS RELATIONSHIP FOR YOUR AIRCRAFT....................................... 5 4. CORRECT ALIGNMENT OF THE PITOT TUBE ....................................................................... 9 APPENDIX A – ASI INSTRUMENT ERROR SHEET ....................................................................... 11 Table of Figures Figure 1 Arrangement of instrument calibration system .......................................................... 3 Figure 2 IAS instrument error sample ........................................................................................ 7 Figure 3 Sample relationship between IAS and CAS for power off descent .............................. 8 Figure 4 Longitudinal alignment of pitot tube ........................................................................... 9 Figure 5 Vertical alignment of pitot tube ................................................................................ 10 Abbreviations and Symbols ASI: Airspeed Indicator CAS: Calibrated airspeed EFIS: Electronic Flight Information System IAS: Indicated airspeed ISA: International Standard Atmosphere Eins: Instrument error Epos: Position error Vqc: Simulated calibrated airspeed iv 1. BACKGROUND The aerodynamic forces acting on an aircraft are directly dependent on the dynamic pressure encountered by the aircraft. In essence, the dynamic pressure is that portion of the total pressure in the atmosphere that acts on the aircraft due to its movement through the air. The calibrated airspeed (CAS) is directly representative of the dynamic pressure acting on the aircraft regardless of altitude, wind, temperature etc. Thus, at a given CAS, the aerodynamic forces acting on the aircraft are constant, regardless of altitude, temperature etc. This conveniently means that the stall CAS for a given aircraft weight and configuration will be much the same within the flight envelope of a typical light aircraft, regardless of the environmental conditions. The pitot-static system installed in the aircraft attempts to display CAS to the pilot but is subject to numerous errors. In order to determine the relationship between indicated airspeed (IAS) and calibrated airspeed (CAS) it is necessary to account for the most significant errors present in the pitot- static system. These errors are instrument error and position error. • Instrument error: The error present in the reading of the instrument due to manufacturing imperfections and wear and tear. This can vary vastly depending on the make, age and condition of the instrument. • Position error: The error present in the reading due to the system sensing pressures that are not truly representative of what the aircraft is encountering. These errors come from the position on the aircraft at which the total and static pressures are sensed and can be affected by the pressure distribution around the aircraft, the propeller slipstream etc. Up to the time of writing, the standard pitot-static system installed in the Cheetah/BushCat made use of a pitot tube to sense total pressure, while the instruments themselves (ASI and altimeter) were left to sense static pressure within the cabin. This Technical Guidance Material aims to assist the owner/operator in generating the approximate relationships between IAS and CAS for their aircraft. Information is entered into the accompanying Excel spreadsheet and the calibration graphs will automatically be generated for your system. You can then print these graphs for reference. These relationships are subject to potential errors present in the tests used to obtain them, but 1 have been shown to be representative of reality (within about 1.5mph in their applicable range) when applied to another aircraft in the fleet. Note that if any changes have been made to the cabin, doors or pitot-tube of the aircraft, these will affect the validity of this method. 2. DETERMINATION OF INSTRUMENT ERROR FOR YOUR ASI The procedure to determine the instrument error present for your ASI is a relatively simple one but must be done carefully and accurately. It requires 2 people to accomplish and involves applying a pressure to the pitot tube to simulated a calibrated airspeed. This will generate a certain indicated airspeed on the instrument. This is done for multiple values all the way up and down the speed scale while recording the pressures corresponding to each indicated airspeed. Upscale and downscale readings are taken as, for example, the needle of the ASI may lag behind the movement of the pressure capsule due to mechanical free play in the linkages. This could mean that the indicated airspeed for a given calibrated airspeed in flight could vary depending on the movement of the linkages in the instrument before reaching that airspeed, and so it is important to determine the magnitude of this effect. Equipment: • Calibrated differential manometer. • Means to apply a pressure to the pitot tube (such as a small syringe). • Rubber/silicone tubing of roughly 4mm internal diameter. • T-piece connector to connect the syringe and manometer to the pitot tube. • Tie-wraps. Method: 1. Connect one end of the differential manometer to the t-piece connector using a length of rubber tubing. Secure a syringe to a free leg of the t-piece connector with another piece of rubber tube. 2. Zero the reading on the manometer. If there is no zeroing function, note the zero-IAS pressure reading as an entry in Column 1 for an IAS of zero. 2 3. Carefully attach the last end of the t-piece connector to the pitot tube itself with a third length of rubber tubing. When attaching the arrangement to the pitot tube, ensure that the plunger of the syringe is inserted and on its lower stop. Use tie-wraps to secure each connection. Figure 1 Arrangement of instrument calibration system 4. Have someone sit in the cabin viewing the ASI as the pilot would in flight. The second person must then slowly apply pressure to the syringe until the needle on the ASI reads the closest round value (usually about 20 to 25mph for analogue instruments). The person in the cabin calls when the desired value has been reached (record this in Column 1 of Appendix A, or the spreadsheet provided). 5. Hold the syringe steady (the friction on the plunger should be sufficient for you to let it go). Let the pressure value stabilise. i. Note that leaks will be evident by a steadily decreasing pressure value on the differential manometer. If the leak rate exceeds approximately 5-10 Pascals per second, it is likely that the pitot line itself has a leak and will have to be changed. ii. If the desired airspeed value on the ASI is overshot (for example, the needle indicates 22mph, rather than the desired 20), the pressure on 3 the system must be reduced to about 5mph below the intended value (15mph for the example given), before pressure is reapplied to obtain the correct value. 6. Record the reading on the differential manometer in Column 2of the sheet provided in the Appendix A. 7. Gradually apply pressure to the syringe to obtain the next speed value on the ASI and repeat the process. Go up in 5mph increments until at least 90mph. A 5ml syringe should be just sufficient to obtain the pressure required for this. 8. Once the highest speed has been obtained, gradually reduce pressure on the syringe until the next round number below it has been obtained. 9. Continue back down the speed scale taking pressure readings at the same values that you took going up the speed scale (completing Column 3, but from bottom to top). i. Note again that if the desired airspeed value on the ASI is overshot while reducing pressure (for example, the needle indicates 83mph, rather than the desired 85), the pressure on the system must be increased to about 5mph above the intended value (90mph for the example given), before pressure is slowly reduced again to obtain the correct value. ii. Once the applied pressure has been reduced to zero (plunger of syringe is completely removed from cylinder) note the zero reading on the manometer again in the zero IAS row. Section intentionally left blank. 4 3. GENERATING THE IAS-CAS RELATIONSHIP FOR YOUR AIRCRAFT Now that the pressure measurements are complete, Columns 1 to 3 of the sheet should be populated with values. This section details the calculation of Columns 4 to 19 in order to generate the relationship for your aircraft. This section requires some calculation and can be done manually with a pocket calculator, or using the spreadsheet provided. 1. Columns 4 and 5: Convert each measured upscale and downscale pressure to the CAS that it corresponds to (denoted as Vqc) using the formula below.
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